Many industrial processes must handle fluids which also include solids in the form of entrained particles or precipitate (i.e., suspended solids). Due to temperature, pressure and other operating condition changes, these solids can deposit on the walls of the fluid process system in the form of scale, causing fluid handling problems and plug subsurface geothermal formations and injection wells. This can require special handling equipment (e.g., filtration) and special procedures, such as periodic back-flush or shutdown for solids removal and rework of injection wells. These added solids handling facilities and procedures can consume significant time, energy, and expense. In addition, facilities and procedures which are cost effective for one type and amount of solids may be ineffective for other types and amounts of solids, e.g., chemical cleaning can be quick and effective for removing a small amount of calcium carbonate-rich scale, but very costly (or even ineffective) for smaller amounts of carbonate in heavier scale deposits.
The ability to accurately forecast and/or measure solids in a fluid handling system (without shutting down the process) would minimize these handling problems and costs. Accurate forecast and measurements would allow optimization of solids removal efforts.
However, fluid process conditions (e.g., elevated temperature and pressure) can cause measurement problems. Limited fluid system access during production or other activities may also be a problem. These measurement problems have typically required sampling, rather than direct in-stream measurement. A typical sampling device is a Pitot tube which diverts a fluid (and suspended solids) to an ambient temperature and pressure collection device. The collected sample is then later analyzed in a laboratory.
In some process industries, such as geothermal energy extraction, these measurement difficulties are greatly compounded. The process fluid (brine) is saturated or supersaturated with scale-forming dissolved solids and acid gases. Usually, energy is extracted from geothermal brine by flashing to produce steam, which is then used to drive a steam drive. The flashing procedure, however, also lowers the pressure and temperature of the remaining geothermal brine which in turn tends to reduce solubility and supersaturate the dissolved solids and liberate gases. The supersaturated solids, liquids, and remaining gases may rapidly undergo chemical changes so that standard sampling and testing techniques generate erroneous results. For example, suspended solids measurement may erroneously include post-sampling precipitation. Still further, the scaling and rapid precipitation can clog or otherwise adversely affect sampling devices and methods.
A fluid sampling device has been developed by Battelle Memorial Institute for the U.S. Department of Energy at Pacific Northwest Laboratory. As disclosed in Technical Report No. PNL3412, UC66d, dated January 1980, Chapter 9 by R. P. Smith, the Battelle suspended solids sampling device is designed specifically for geothermal fluids. The device obtains a fluid and suspended solids sample in a Pitot tube-like probe and conduit. The sample conduit diverts the fluid sample out of the process flow stream to a heat exchanger to quickly cool the fluid (tending to stabilize the mixture by reducing the rapid rate of chemical reaction and precipitation). After cooling, the device filters the sample to remove the suspended solids. Any remaining liquid sample is then collected for laboratory analysis.
Although the rapid precipitation rate is reduced and suspended solids are removed by the Battelle sampling device, measurement problems remain. Diverting the suspended solids in the sample line may deposit suspended solids before collection and accelerate precipitation reactions. Sample cooling slows but does not stop precipitation. Sample cooling thermal gradients may also cause further deposition of the sampled suspended solids before filtration. Thus, even if the fluid (and entrained suspended solids) sample is initially representative, the sample quickly becomes unrepresentative.
Another measurement approach is to provide a process fluid side stream. The side stream is periodically isolated (without disturbing the main process system) and fluid samples or coupons removed for measurements. Coupons or other surfaces in the side stream are exposed to flowing brine to represent main process system surfaces under process conditions. After exposure to the coupons, the side stream fluids may be returned to the main stream or separately discharged.
However, side stream measurements have also been inaccurate. Side stream fluid conditions cannot fully duplicate main stream conditions. Diversion to the side stream may again cause unrepresentative scaling/precipitation. Although brine is flowing, side stream geometry is different (e.g., smaller side stream geometry can provide more pipe contact area for precipitation than main process stream). Flow distribution conditions are also different from the process stream. These differences can affect precipitation, scale formation and solids measurement.
Besides these problems, the suspended solids measurement device and method must also be able to handle a variety of process conditions. A sampled or side stream measurement may be nearly representative at certain fluid process conditions, but not at others, such as part load operation. The measurement device should also be capable of providing rapid measurements in response to changes in process conditions.
None of the current approaches known to the inventors eliminate the aforementioned side stream and sample collection problems. In addition, the problem of periodic solids removal from side stream or sampling apparatus remains.